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LTC2932I

Confi gurable Six Supply Monitor with Adjustable Reset Timer and Supply Tolerance

Linear Technology 

LTC2932

APPLICATIONS INFORMATION

Supply Monitoring

The LTC2932 is a low power, high accuracy configurable

six supply monitoring circuit with six real-time monitor

outputs, a common reset output, a reset disable function

and selectable supply thresholds. An external capacitor

sets the reset timeout period. An external resistive divider

between VREF, VPG and GND selects 1 of 16 possible input

voltage monitor combinations. All six voltage inputs must

be above their predetermined thresholds for the reset not to

be activated. The LTC2932 asserts the reset and compara-

tor outputs during power-up, power-down and brownout

conditions on any one of the voltage inputs.

NOMINAL

5V

SUPPLY

SUPPLY TOLERANCE

VOLTAGE

MINIMUM

RELIABLE

SYSTEM

VOLTAGE

IDEAL

SUPERVISOR

THRESHOLD

4.75V

±1.5%

THRESHOLD 4.675V

BAND

4.6V

REGION OF POTENTIAL MALFUNCTION

–5%

–6.5%

–8%

Power-Up

The greater of V1 and V2 serves as the internal supply

voltage (VCC). On power-up, VCC powers the drive circuits

for the RST pin. This ensures that the RST output will be

low as soon as either V1 or V2 reaches 1V. The RST output

remains low until the part is configured. Once the voltage

thresholds are set, if any of the supply monitor inputs is

below its configured threshold, RST will be a logic low.

Once all the monitor inputs rise above their thresholds,

an internal timer is started and RST is released after the

delay time. If VCC < (V3 – 1.0V) and VCC < 2.4V, the V3

input impedance will be low (10kΩ typical).

Threshold Accuracy

Consider a 5V system with ±5% tolerance. The 5V supply

may vary between 4.75V to 5.25V. System ICs powered by

this supply must operate reliably within this band (and a little

more as explained below). A perfectly accurate supervisor

for this supply generates a reset at exactly 4.75V, however

no supervisor is this perfect. The actual reset threshold

of a supervisor varies over a specified band; the LTC2932

varies ±1.5% around its nominal threshold voltage (see

Figure 1) over temperature.

The reset threshold band and the power supply tolerance

bands should not overlap. This prevents false or nuisance

resets when the power supply is actually within its speci-

fied tolerance band.

Figure 1. 1.5% Threshold Accuracy Improves System Reliability

The LTC2932 has a ±1.5% reset threshold accuracy, so a

“5%” threshold is typically set to 6.5% below the nominal

input voltage. Therefore, a typical 5V, “5%” threshold is

4.675V. The threshold is guaranteed to lie in the band be-

tween 4.750V and 4.600V over temperature. The powered

system must work reliably down to the low end of the

threshold band, or risk malfunction before a reset signal

is properly issued.

A less accurate supervisor increases the required system

voltage margin and increases the probability of system

malfunction. The LTC2932’s ±1.5% specification improves

the reliability of the system over supervisors with wider

threshold tolerances.

Monitor Configuration and Tolerance Selection

Select the LTC2932 input voltage combination by placing

the recommended resistive divider from VREF to GND and

connecting the tap point to VPG, as shown in Figure 2.

Table 1 offers recommended 1% resistor values for each

of the 16 modes. The last column in Table 1 specifies

optimum VPG/VREF ratios (± 0.01), when configuring with

a ratiometric DAC.

LTC2932

VREF

VPG

GND

R1

1%

R2

1%

2932 F02

Figure 2. Mode Selection

2932fb

9

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